An image sensor includes an array of sensor elements to receive and detect electromagnetic radiation. The sensor elements generate electronic signals in response to the electromagnetic radiation with amplitudes indicative of the intensity. A sensor element may include a semiconductor device such as a charge-coupled device (CCD), photodiode array, charge injection device, or metal oxide semiconductor (CMOS). CCD sensor arrays are a proven technology that have been commercial available for many years. A CMOS sensor element includes multiple transistors for each pixel. An active pixel sensor (APS) array is a CMOS sensor array with radiation sensing circuitry and active circuitry designed in each pixel. Active circuitry includes interconnected transistors that are in an opaque area that is not used for radiation sensing. Accordingly, each pixel includes radiation sensitive and non-radiation sensitive circuitry.
Dark current, also referred to as noise, is a thermal excitation in a sensor element that is not generated by received electromagnetic radiation. The thermal excitation releases electrons into individual sensor elements in a sensor device. The thermally released electrons are collected along with electrons that are released by photon excitation. A sensor is not able to distinguished from thermally excited electrons and electrons released by incoming photons. This creates element values that are too high when the image is read out of a sensor device.
All image sensors are subject to dark current. However, in most digital applications the amount of photo-electrically generated electrons is overwhelming compared to the number of thermally released electrons. Thus, dark current is often a mostly ignored phenomenon. In some circumstances, such as astronomical photography, the amount of incoming light is usually quite small. This makes it necessary to take long exposure photographs. With both consumer cameras and astronomical cameras, the low light levels combined with the long exposure times puts the dark current on an order of magnitude with the photon excited current thereby making dark current a significant factor.
Correcting for dark current involves taking a dark frame image, which is an image with the shutter closed. The sensor element values of the dark frame are then subtracted from the original image to obtain the true value. The additional steps of taking a dark frame and then subtracting the dark current values are tedious and are subject to error. It would therefore be advantageous to provide accurate dark current correction without additional user action.